1. Field of the Invention
The present invention relates to a grooved-inner-surface heat transfer tube with fins for enhancing heat exchange efficiency formed in a zigzag shape on the inner surface of a metal tube.
2. Description of the Related Art
This type of grooved-inner-surface heat transfer tube is used mainly as an evaporator tube or a condenser tube in heat exchangers or the like in air-conditioners or cooling apparatus. Recently, heat transfer tubes formed with spiraling fins over the entire face of the inner surface have been widely marketed.
The heat transfer tubes which are currently most popular are manufactured by a method wherein fins are roll-formed over the entire inner surface of a metal tube by passing a floating plug formed with spiral grooves on the outer circumferential surface, along the interior of a seamless tube (with no joins) obtained by a drawing or an extrusion process.
In grooved-inner-surface heat transfer tubes formed with spiral fins of this type, heat transfer liquid which has collected to the bottom of the interior of the heat transfer tube is drawn up along the spiral fins by being blown by a vapor current flowing inside the tube, thereby spreading along the entire circumferential surface inside the tube. Due to this effect, the entire circumferential surface inside the tube is made almost uniformly wet, so that in cases where the tube is used as an evaporation tube for evaporating the heat transfer liquid, the area of the region where boiling occurs can be increased to improve the boiling efficiency. Additionally, in cases where the tube is used as a condenser tube for liquefying heat transfer gas, the condensation efficiency can be increased by increasing the contact efficiency between the metal surfaces and the heat transfer gas due to the tips of the fins being exposed from the surface of the liquid.
However, it is apparent that there is room for further improvement of the effect of increased heat transfer efficiency due to the spiral fins. It is for this reason that the inventors of the present invention have applied electric resistance welding processing methods for the manufacture of heat transfer tubes and have produced many types of grooved-inner-surface heat transfer tubes by changing the patterns of the grooves in the heat transfer tubes, and then carried out experiments to compare their performance. As a result, they discovered that better heat transfer performance can be obtained in comparison to simple spiral patterns and the like, if multiple fins are formed extending in a zigzag pattern around the circumferential direction of the inner surface of the heat transfer tubes.
However, in cases where this type of zigzag patterned fins were formed, there was another problem of a substantial pressure loss due to fluid resistance applied to the fluid when the fluid flows over the bend portions of each of the fins having a convex shape toward the downstream direction.
In order to alleviate this increase in pressure loss, a trial was made involving forming a gap in each of the bend portions of the zigzag patterned fins to have the fluid flow through each of the gaps. However, while there was some reduction in the pressure loss, heat exchange efficiency decreased markedly, so that there arose the problem of not being able to obtain the high heat exchange efficiency of the conventional zigzag patterned fins.